Real Estate Real Estate Properties Acquired (Details) - Operating Segments [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) a ft² bldg	Dec. 31, 2014 USD ($) a ft² bldg
Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	758	502
Land and Land Improvements [Member] | Carolinas [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a		104
Land and Land Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a		16
Land and Land Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	139	110
Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	281	85
Land and Land Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	188	24
Land and Land Improvements [Member] | Richmond Hampton Roads [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	94
Land and Land Improvements [Member] | South Florida [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a		69
Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land (unaudited) | a	56	94
2015 acquisitions [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 62,402
2015 acquisitions [Member] | Land and Land Improvements [Member] | Richmond Hampton Roads [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 1,727
2015 acquisitions [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	5
Leaseable square feet (unaudited) | ft²	1,529,211
2015 acquisitions [Member] | Building and Building Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	3
Leaseable square feet (unaudited) | ft²	921,196
2015 acquisitions [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	0
Leaseable square feet (unaudited) | ft²	0
2015 acquisitions [Member] | Building and Building Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	1
Leaseable square feet (unaudited) | ft²	197,956
2015 acquisitions [Member] | Building and Building Improvements [Member] | Richmond Hampton Roads [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	0
Leaseable square feet (unaudited) | ft²	0
2015 acquisitions [Member] | Building and Building Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	1
Leaseable square feet (unaudited) | ft²	410,059
2015 acquisitions [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 220,415
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Purchase price | $	86,497
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Purchase price | $	24,541
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 45,248
2014 acquisitions [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		10
Leaseable square feet (unaudited) | ft²		1,928,608
2014 acquisitions [Member] | Building and Building Improvements [Member] | Carolinas [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		7
Leaseable square feet (unaudited) | ft²		947,365
2014 acquisitions [Member] | Building and Building Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet (unaudited) | ft²		0
2014 acquisitions [Member] | Building and Building Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet (unaudited) | ft²		0
2014 acquisitions [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet (unaudited) | ft²		0
2014 acquisitions [Member] | Building and Building Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet (unaudited) | ft²		0
2014 acquisitions [Member] | Building and Building Improvements [Member] | South Florida [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet (unaudited) | ft²		0
2014 acquisitions [Member] | Building and Building Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		3
Leaseable square feet (unaudited) | ft²		981,243
2014 acquisitions [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 212,193
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Carolinas [Member]
Business Acquisition [Line Items]
Purchase price | $		61,293
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Purchase price | $		7,914
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Purchase price | $		26,210
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $		15,307
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Purchase price | $		2,966
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | South Florida [Member]
Business Acquisition [Line Items]
Purchase price | $		12,027
2014 acquisitions [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 86,476